This application relates to a method for removing methanol from vent gas streams generated in a cumene-to-phenol process.
In the well known cumene-to-phenol process, methanol is produced as a minor by-product during the cumene oxidation reaction step. Several reactor designs exist in the industry today in which the reaction is typically conducted at 70-130xc2x0 C. and 20-120 psig. In these processes, air is continuously introduced into liquid cumene contained in multiple gas-sparged towers or tanks. The oxygen in the air serves as raw material to produce the desired intermediate product, cumene hydroperoxide. The nitrogen contained in the air feed stream along with 1-10% residual oxygen exits the top of each of the reactors. This xe2x80x9cspent airxe2x80x9d stream is saturated with water, cumene and contains other trace volatile organic compounds (VOC""s) including methanol. Prior to venting, the current standard practice is to pass this xe2x80x9cspent airxe2x80x9d stream through a series of heat exchanger coolers, refrigerated condensers and activated carbon adsorption beds to recover the cumene value contained therein and to prevent VOC emissions to the environment.
It is widely recognized in the industry that the activated carbon adsorption beds mentioned above are ineffective in removing the methanol component and a majority of the methanol contained in the xe2x80x9cspent airxe2x80x9d stream passes through the beds unadsorbed. Also the vent gas coolers and refrigerated condensers provided in the standard designs are not effective in condensing the methanol from the xe2x80x9cspent airxe2x80x9d vent gas stream due to its high volatility and due to the fact that the very large volume xe2x80x9cspent airxe2x80x9d stream passes through these shell-and-tube exchangers at very high velocity resulting in poor heat transfer and poor condensation efficiency. This serious hazardous air pollutant (HAP) emission problem impacts all cumene-to-phenol producers today; and expensive and complicated downstream control devices must be installed in these plants to adequately control and reduce methanol emissions. Such VOC control devices include expensive thermal incinerators, catalytic oxidation incinerators and special gas-scrubbing columns.
U.S. Pat. No. 5,891,411 (Gribbon) and U.S. Pat. No. 5,375,562 (Brinck) decribe catalytic combustion methods utilizing a regenerative catalytic oxidizer unit for purifying exhaust gases from chemical processes to remove volatile organic compounds including methanol. This process is effective but requires a high capital investment for the unique equipment, including special multi-pass heat exchangers and catalyst. This method also risks the formation of undesirable NOX pollutant due to the high operating temperatures required. U.S. Pat. No. 5,907,066 (Wachs) describes the use of a special metal oxide catalytic process to convert methanol in waste vent gas to formaldehyde. This method is also costly due to catalyst plus the conversion of methanol to formaldehyde is not desirable because it is a harmful VOC pollutant.
U.S. Pat. No. 5,891,410 (Modic) describes a process for the purification of an off-gas containing methanol which originates from oxidation of xylene with air. However in this case special solvents are employed and expensive countercurrent multi-stage extraction columns are required in order to remove the methanol to low levels. Similar and even more complicated and expensive scrubber-extraction systems for removal of VOC""s from vent gases using water are described in U.S. Pat. Nos. 5,186,728, 4,948,402 and 4,734,108. All of these methods suffer due to high investment cost coupled with complicated operation.
Thus, there remains a need for process improvements which will facilitate the removal of methanol from cumene-to-phenol process vent gases at reasonable cost.
It has now been surprisingly discovered that injection of even very small amounts of supplemental water to the spent-air stream at points upstream from the heat exchanger cooler(s) allows these heat exchanger coolers to act as both extractors and condensers. It is therefore possible to recover from the heat exchanger cooler a methanol/water condensate, thereby substantially reducing the amount of methanol in the spent-air stream prior to discharge. Thus, the invention provides an improvement to known methods for manufacture of phenol from cumene in which an oxygen-containing gas stream is passed through liquid cumene to produce an oxidate product and a spent-air stream comprising methanol and cumene and a saturating amount of water; and in which the spent-air stream is passed through one or more heat exchanger coolers and a carbon bed prior to discharge. The improvement comprises the steps of injecting a supplemental amount of water into the spent air stream at one or more points upstream from at least one of the one or more heat exchanger coolers, and recovering a methanol/water condensate from the heat exchanger coolers, thereby reducing the methanol content of the spent-air stream prior to discharge.